Many gases cannot be detected in the IMS because of the presence of water vapor and other interferences normally present in the current state of the art for the IMS instrument. The heretofore undetectable gases in the IMS can now be detected by practicing methods and by using apparatus which will reduce the water vapor and other interferents by the use of purification materials and apparatus described in this invention.
A number of methods are known for the purification of gases. Molecular sieves are porous, inorganic materials which present a large clean, very porous surface that can remove water and organic vapors from a gas stream, mainly by physical adsorption. This porous material operates at room temperature, but it is better and has a larger adsorptive capacity at lower temperatures. It is reactivated by heating in a clean gas stream into which it will give off its adsorbed materials. The molecular sieve has an adsorptive efficiency which is a function of many parameters including the impurities adsorbed.
Activated charcoal works broadly in the same general manner as molecular sieve. It is less efficient with water but adsorbs vapor organics better as in the case of gas masks and the like. Again, adsorption properties for specific materials vary among the charcoals, based on the charcoal source, preparation, activation, recycling, etc.
Cryogenic purification adds cooling of the adsorbent. Reducing the temperatures of the material improves the retention capacity for the impurities where physical adsorption mechanisms are involved.
Getters involve a method of converting a gas to a solid by the use of chemical reaction of the impurity and the getter material. As an example, porous zirconium reacts, when heated, with all materials except rare gases. Thus rare gases (helium, argon, etc.) can be purified. Getters are usually used in a vacuum but since they do not absorb rare gases, they can be used to purify these substances also without vacuum (see for example Handbook of Electron Tube and Vacuum Technique, Rosebury Publ., 1965, p. 105).
One variety of gas chromatograph column is filled with the adsorbing, purifying material (molecular sieve, charcoal, etc.) to form a tube which is elongated as compared to its diameter. An already cleaned rare gas, e.g. helium, is used as the carrier gas in the column. A gas sample is added to the column (as an injection made at the head of the column) which sample can be considered a mixture of the target gas and its impurities. Sorting of the gases occurs wherein the target gases have the shorter retention time and the impurities the longer retention time as they pass through the column. Thus the impurities are removed from the mixture by remaining in the column. The column is cleaned by back flushing with clean gas and venting the impurities. In the present case where water vapors constitute the undesired impurities, the water has the long retention time and remains in the column. In this manner a dry target gas sample exits from the column, and can be introduced into the IMS system.
Selective membranes can also be used for gas purification. These materials utilize differential solubility of vapors in the membrane and/or differential diffusivity of vapors.
There is an increased commercial need for the measurement of gases that are present in trace amounts, typically in concentrations of 100 parts-per-billion and less. Ion mobility spectrometry (also referred to in the prior an as plasma chromatography) is a particularly useful technique for the determination of analyte gases in trace amounts. Typical uses of IMS are is described, among others, in earlier U.S. Pat. Nos. 3,262,180; 3,262,182; 3,593,018; 3,596,088; 3,621,239; 3,621,240; 3,624,389; 3,626,178; 3,626,179; 3,639,757; 3,668,382; 3,668,385; 3,697,748; 3,699,333; 3,742,213; 3,812,355; 3,845,301; 4,195,513; and 5,162,652, and further in U.S. Pat. Nos. 3,262,181;, 3,629,574; and 3,668,383. In IMS the trace chemicals to be detected are ionized and then separated from each other due to their differing drift velocities in an electric field. The time differentials of their arrivals to a collector are then registered. Hence the nomenclature referring to "ion mobility" in the spectrometric procedure.
FIG. 1 shows the basic operating characteristics of an ion mobility spectrometer, such as the commercially available model MMS 160 Ion Mobility Spectrometer/Quadrupole Mass Spectrometer system (IMS/MMS) sold by PCP, Inc., West Palm Beach, Fla., under the trademark Phemto-Chem.RTM.. In this model the IMS is coupled to an optional quadrupole mass spectrometer through a 30.mu. aperture for further analysis under high vacuum of the ions from the IMS collector. The operation of such a device is well known and described in the literature, such as in the book Plasma Chromatography, edited by T. W. Carr, Publ. Plenum Press, New York 1984.
An IMS is a chemical vapor detector operating at atmospheric pressure. It can be used with a variety of gases, such as air, nitrogen, argon, helium, etc. A vapor sample can be introduced in many ways into the IMS, in which a radiation source, for example a nickel-63 .beta.-radiation source in the MMS 160 IMS/MMS, ionizes the host gas at atmospheric pressure. This primary ionization initiates a sequence of ion-molecule interactions which lead to the formation of sufficiently energetic positive or negative ions which, in turn, ionizes the constituents of the vapor sample. All of the ions move downstream in the IMS cell under the influence of an applied voltage gradient and are separated in the drift region of the cell based on the unique drift times of the various chemical ion species, which generate the ion mobility spectrum.
An IMS utilizes a drift gas generally to provide a gaseous environment wherein the ions produced in the reaction region of the IMS in the carrier gas can drift with no change in identity which could arise from the continuation of ion-molecule reactions which have taken place in the reaction region of the IMS.
Humanity lives in a atmosphere containing a substantial amount of water vapor. Therefore, the most common and plentiful IMS interferant on earth is water. In extraterrestrial environments other vapors, such as methane or ammonia may be the most common interferant. Therefore, all references herein to water are meant to include other atmospheric IMS interferants, given the nature of the predominant atmosphere at the site of use.
Water vapor present in conventional IMS tends to react with the energetic ions extremely rapidly, such as within a millisecond, through ion-molecule interactions, thus precluding measurement of the energetic ions mobility in conventional apparatus and by conventional techniques.
Normally the energetic ions react quickly with the water to become the hydrated proton (H.sub.2 O).H.sup.+. Therefore, the higher concentration of the water, the larger the value of n. This results in the problem that a large number of gaseous chemicals are not detectable, because their charge has transferred to water as the ionic product. In this water vapor the detectable product ions are those to which the positive charge is finally transferred from the hydrated water ion to produce the observed ion mobility spectrum.
Current IMS technology utilizes gases usually containing at least 10 parts per million of water vapor. This water concentration dominates the ion-molecule interaction sequence to produce in the positive ion mode, the water cluster reactant ion and the observed ion mobility spectrum. This water reactant ion does not tend to ionize some sample molecules that are intended to be detected, such as saturated hydrocarbons. Other chemical traces which could not be detected by the IMS were the ions of the so-called "energetic" ions, because as soon as they were formed, they quickly reacted to ionize less energetic gases, such as water vapor. For example, helium, oxygen, and nitrogen (which are the host gas components present at large concentrations) are some of the most energetic species due to the larger amount of the energy required to ionize these neutral molecules. Then the host gas ions quickly react with the lower concentration of water which would be normally present in the moist host gas. The water ions and water cluster ions interact with less energetic trace chemical species which have relatively low ionization potentials, or high proton affinities, to produce the trace chemical ions seen usually in the normal standard ion mobility pattern. Thus in normal, standard IMS the water ion is the reactant for these lower energy chemicals.
In view of the ubiquitous presence of water in the atmosphere, water vapors are the most common interferant in ion mobility spectrometry. Water, however, is not necessarily the only interferant. Under particular circumstances, other interferants could be any neutral species present in an excessive amount, such as oxygen, hydrocarbons, etc. As a rule of thumb, any species in excess of 0.1 ppm can interfere with the measurement of trace gases which are present the concentration of pans-per-billion and lower. Therefore, while reference is made herein generally to water since it is the dominant example of interferants, and since depending on its concentrations also the most simply measurable target species, such reference is also intended to include all other interferants, particularly those which are present in concentrations of 2 and more orders of magnitude higher than the concentration of the trace gases to be detected.
In conventional IMS technology, even when taking the best precautions, it is difficult to obtain gases with a total impurity level of 1 part per million. Therefore, a total impurity of 1 part per million is considered "ultra-pure" in IMS of current common commercial designs. The lifetime of an ion which is required to produce a mobility peak is in the order of magnitude of 10 to 20 milliseconds, or longer. This lifetime can be obtained by reducing the concentration of any would-be neutral reactant interferant to a level where a negligible amount of its product ion is formed. If, however, the would-be reactant is water, then its concentration in IMS must be at or below low parts per billion range to be not seen in the ion mobility pattern. With the method and means of this invention such conditions have been constructed and observed.